The invention relates to automatic gauge control of rolling mills, in general, and more particularly to a system for calibrating the roll force applied to the roll stands of a rolling mill.
Automatic gauge control of thickness reduction through one or more stands of a rolling mill rests upon a mass flow relationship between speed and thickness through all the stands. Actual performance of a roll stand in relation to speed and roll force depends upon the initial setting of the roll stand from an empty and unloaded condition to a well-identified preloaded condition to throughout the transformation process. The schedule may be calculated by a computer. If, however, a computer is not used, the mill setup must be made by the mill operator. In such case, a precise and easily performed mill setup with the assist of an operator could be of much benefit on non-computer mills having a sophisticated system of hydraulic automatic gauge control, by advantageously providing additional "on-gauge" performance. Moreover, an improved mill setup system would add, to the many hot strip mills in existence for many years, the capability of producing products having a quality comparable to what is now obtained with many of the newer mills. The existing manufacturers would thus be able to extend the useful life of their manufacturing equipment without having to invest in more modern rolling mill installations.
For mills which have fast acting roll gap actuators, such as hydraulic cylinders, it is possible and practical to calibrate the stand roll gaps every time that a schedule change is to be made. One method of calibrating is to:
(1) predict the stand roll force for the new schedule.
(2) close the roll up gap below face to obtain the predicted roll force with no strip in the mill.
(3) open the roll gap from this preloaded position by exactly the thickness of the desired delivered strip thickness from this stand.
The above setup procedure will produce on gauge strip provided the predicted roll force is correct and also provided the mill modulus with strip in the mill is the same as the mill modulus with no strip through the mill stands.
In any given work roll position relative to strip material being passed through the stand, there is a stand roll force RF, a stand screwdown or hydraulic cylinder position S establishing a roll gap opening H for the processed materials and these parameters are related by the formula. EQU H=RF/M+S
where M is the known mill spring modulus of the rolling stand.
When a change in the roll gap opening takes place, the differential relation is as follows: EQU .DELTA.H=.DELTA.RF/M+.DELTA.S.
While the workpiece delivery gauge is determined by the equilibrium point where the roll force of the rollstand is equal to the force required to deform the product, several factors intervening as changing parameters may make the basic equation invalid. For instance the plasticity of the strip material may have to be taken into account. This factor has been given consideration in the rolling gauge control mill method and apparatus of U.S. Pat. No. 3,709,008 of A. W. Smith. Another factor is the working roll curvature. The shape and the thickness of the workpiece strip delivered from a rolling mill are determined, according to U.S. Pat. No. 3,404,550 of R. G. Plaisted, by a controlled sending of the work rolls, and any workpiece strip shape error is corrected in order to obtain the desired delivery thickness.
Instead of using as in the Plaisted patent, bending force transducers to generate a control signal effective on a bending force control device together with bending compensation operative on the roll positions, the present invention proposes to effectuate correction at the preloading stage of the stand in such a way as to take into account the strip width which is a major factor in causing the work rolls to bend, particularly when the strip width is narrow. In this respect, it is realized that the applied roll force is not distributed over the entire width of the upper work roll, whereas the back-up roll maintained at two ends by its support tends to deflect the rolls at the extreminities. As a result, the mill modulus is M.sub.2 which is different from the mill modulus M.sub.1 when the stand is empty.
In this respect, it is realized that the roll force from the screwdown or hydraulic cylinder is not distributed over the entire length of the upper work roll when the material strip is narrow, is interposed in the gap only toward the center of the rolls, whereas the support or reacting forces applied on the arcs of the lower roll, are toward the extremities, tending to deflect the bottom roll from the ends, thereby causing roll bending.
It has been shown, for instance in the aforementioned U.S. Pat. No. 3,709,008 of A. W. Smith, Jr., that the mill modulus of the rolling stand is a constant used by automatic gauge control systems in estimating the roll force and the gauge, and that such constant is the inverse slope of the mill characteristic curve. The mill modulus is known to be affected by the width of the strip of material being rolled. Accordingly, when setting up a stand on an empty mill, a change of modulus can be expected as a function of strip width. Therefore, for a given total roll force estimated, a change in strip width will result in a change in the deflection of the rolls, even if the mill housing deflection should remain constant. The total mill deflection is the sum of all the deflections experienced by the several mill stand parts, the total discrepancy could be quite substantial. It is known from experience, as well as from calculations, that variations in strip width from maximum to minimum width may affect the total modulus by as much as 15% to 25% depending upon mill width and roll diameters.
The present invention provides for automatic gauge control of a rolling mill in which the mill can be set up so that variation in the mill modulus as a function of width of the strip is automatically computed and compensated for.
The present invention also provides for automatic zero-correction of error due to strip width variation in the initial setup of a rolling mill.
The present invention moreover provides an improved and easier setup of a rolling stand for a better and more precise performance under automatic gauge control.
The present invention further improves the operation of an electrohydraulic roll force control system of a rolling mill through the provision of a better and more precise setup system.